The overall objective of the research is to gain an understanding of the effect of level of protein intake on calcium metabolism in man. All research will be conducted on human subjects. Efforts will be made to study the physiological mechanisms whereby high protein diets cause hypercalciuria. Four population groups, namely 1) young adult males, 2) older males and 3) pre-and 4) postmenopausal women will be studied. Subjects will be fed carefully controlled diets for varying lengths of time and will be given both low (about 45 to 50 g) and high protein diets (about 130 to 140 g). Calcium, phosphorus and magnesium will be about 500, 800 and 350 mg, respectively, and will be the same at both protein intakes; balances for three minerals will be determined. Specific project (1) is designed to study the mechanisms of protein-induced hypercalciuria in young males, the group in which it is known that increases in protein intake raise urinary calcium. Parathyroid function will be assessed directly by measuring serum parathyroid hormone levels, and indirectly by determining urinary cyclic AMP. Other analyses indicative of calcium metabolism will also be made. Renal function will be examined by measuring glomerular filtration rate, calcium clearance, fractional excretion of calcium and fraction reabsorption. Specific project (2) will determine if high protein diets also cause hypercalciuria in the pre- and postmenopausal female and, if so, a study of the mechanisms will be made.